Because electrolyte solution in a container of a vented battery reduces due to electrolysis of water and evaporation during use, the vented battery needs to be regularly checked for an electrolyte level and needs to be replenished with purified water. For example, for use in an electric vehicle such as a forklift, an assembled battery formed by a plurality of batteries by arranging the plurality of batteries of electromotive force of 2 V, housing them in a housing box, and connecting them in series to obtain electromotive force of 48 V is used, and adding water to the respective batteries is very troublesome. Therefore, conventionally, water addition plugs attached to the respective batteries are connected by tubes to add water to the batteries at once. There is a water addition plug for collective water addition, which has a function of automatically stopping the addition of the water when a prescribed electrolyte level is restored.
Patent Document 1 describes a water addition plug for a storage battery and having, which has an automatic valve function. In this valve mechanism, a water addition port (corresponding to a drain port in the present invention) is formed in a ceiling of a valve chest in a plug. When an electrolyte level rises to a prescribed level, a valve (corresponding to a valve element in the present invention) interlocked with a float closes the water addition port from below (from a side of the valve chest). Once the valve is closed, the valve is pushed against the water addition port due to water pressure in the valve chest to keep a closed state.
Patent Document 2 describes a water addition plug having drain ports in upper and lower portions of a valve chest. Two valve elements are fixed to a shaft connected to a float and substantially simultaneously close the upper and lower drain ports from below when an electrolyte level rises to a prescribed level.